Conventionally, discrete cosine transform (DCT), which is a kind of orthogonal transform coding, has been used as a coding system for efficiently compression-coding still picture data and dynamic picture data. In handling such digital signals on which orthogonal transform has been carried out, it is sometimes necessary to change the resolution or transform base.
For example, in the case where a first orthogonally transformed digital signal having a resolution of 720×480 pixels as an example of home digital video format is to be converted to a second orthogonally transformed digital signal having a resolution of 360×240 pixels of a so-called MPEG1 format, inverse orthogonal transform is carried out on the first signal to restore a signal on the spatial domain, and then transform processing such as interpolation and thinning is carried out to perform orthogonal transform again, thus converting the first signal to the second signal.
In this manner, it is often the case that the orthogonally transformed digital signal is inversely transformed once to restore the original signal, then processed by required transform operations, and then orthogonally transformed again.
FIG. 28 shows an exemplary structure of a conventional digital signal processing device for carrying out resolution conversion as described above with respect to digital signals on which DCT has been carried out.
In this conventional digital signal conversion device, a video signal (hereinafter referred to as DV video signal) of a so-called “DV format”, which one format of home digital video signals, is inputted as a digital signal of a first format, and a video signal (hereinafter referred to as MPEG video signal) of a format in conformity to the so-called MPEG (Moving Picture Experts Group) standard is outputted as a digital signal of a second format.
A de-framing section 51 is adapted for cancelling framing of the DV video signal. In this de-framing section 51, the DV video signal framed in accordance with the so-called DV format is restored to a variable-length code.
A variable-length decoding (VLD) section 52 carries out variable-length decoding of the video signal restored to the variable-length code by the de-framing section 51. The compressed data in the DV format is compressed at a fixed rate so that its data quantity is reduced to approximately ⅕ of that of the original signal, and is coded by variable-length coding so as to improve the data compression efficiency. The variable-length decoding section 52 carries out decoding corresponding to such variable-length coding.
An inverse quantizing (IQ) section 53 inversely quantizes the video signal decoded by the variable-length decoding section 52.
An inverse weighting (IW) section 54 carries out inverse weighting, which is the reverse operation of weighting carried out on the video signal inversely quantized by the inverse quantizing section 53.
The weighting operation is to reduce the value of DCT coefficient for higher frequency components of the video signal by utilizing such a characteristic that the human visual sense is not very acute to a distortion on the high-frequency side. Thus, the number of high-frequency coefficients having a value of 0 is increased and the variable-length coding efficiency can be improved. As a result, the quantity of arithmetic operation of the DCT transform can be reduced in some cases.
An inverse discrete cosine transform (IDCT) section 55 carries out inverse DCT (inverse discrete cosine transform) of the video signal which is inversely weighted by the inverse weighting section 54, and thus restores the DCT coefficient to data of the spatial domain, that is, pixel data.
Then, a resolution converting section 56 carries out required resolution conversion with respect to the video signal restored to the pixel data by the inverse discrete cosine transform section 55.
A discrete cosine transform (DCT) section 57 carries out discrete cosine transform (DCT) of the video signal which is resolution-converted by the resolution converting section 56, and thus converts the video signal to an orthogonal transform coefficient (DCT coefficient) again.
A weighting (W) section 58 carries out weighting of the video signal which is resolution-converted and converted to the DCT coefficient. This weighting is the same as described above.
A quantizing (Q) section 59 quantizes the video signal weighted by the weighting section 58.
Then, a variable-length coding (VLC) section 60 carries out variable-length coding of the video signal quantized by the quantizing section 59 and outputs the resultant signal as an MPEG video signal.
The above-described “MPEG” is an abbreviation of the Moving Picture Experts Group of ISO/IEC JTC1/SC29 (International Organization for Standardization/International Electrotechnical Commission, Joint Technical Committee 1/Sub Committee 29). There are an ISO11172 standard as the MPEG1 standard and an ISO13818 standard as the MPEG2 standard. Among these international standards, ISO11172-1 and ISO13818-1 are standardized in the multimedia multiplexing section, and ISO11172-2 and ISO13818-2 are standardized in the video section, while ISO11172-3 and ISO13818-3 are standardized in the audio section.
In accordance with ISO11172-2 or ISO13818-2 as the picture compression coding standard, an image signal is compression-coded on the picture (frame or field) basis by using the correlation of pictures in the time or spatial direction, and the use of the correlation in the spatial direction is realized by using DCT coding.
In addition, this orthogonal transform such as DCT is broadly employed for various types of picture information compression coding such as JPEG (Joint Photographic Coding Experts Group).
In general, orthogonal transform enables compression coding with high compression efficiency and excellent reproducibility by converting an original signal of the time domain or spatial domain to an orthogonally transformed domain such as the frequency domain.
The above-described “DV format” is adapted for compressing the data quantity of digital video signals to approximately ⅕ for component recording onto a magnetic tape. The DV format is used for home digital video devices and some of digital video devices for professional use. This DV format realizes efficient compression of video signals by combining discrete cosine transform (DCT) and variable-length coding (VLC).
Meanwhile, a large quantity of calculation is generally required for orthogonal transform such as discrete cosine transform (DCT) and inverse orthogonal transform. Therefore, there arises a problem that resolution conversion of video signals as described above cannot be carried out efficiently. Also, since errors are accumulated by increase in the quantity of calculation, there arises a problem of deterioration in signals.